Transboundary secondary organic aerosol in western Japan indicated by stable carbon isotope ratio of low volatile water-soluble organic carbon and signal at m/z 44 in organic aerosol mass spectra

Field studies were conducted in the winter of 2010 at two rural sites and an urban site in western Japan, and filter samples of total suspended particulate matter were collected every 24-h and analyzed for concentration and stable carbon isotope ratio (delta13C) of low volatile water-soluble organic carbon (LV-WSOC). Concentration of major chemical species in fine aerosol (<1.0 micron) was also measured in real time by Aerodyne aerosol mass spectrometers. Oxidation state of organic aerosol was evaluated using the proportion of signal at m/z 44 (fragment ions of carboxyl group) to the sum of all m/z signals of organic mass spectra (f44). Analyses show a high correlation between LV-WSOC and m/z 44 concentrations, suggesting that the LV-WSOC is likely associated with water soluble carboxylic acids in the fine aerosol. Plots of delta13C of LV-WSOC versus f44 exhibit systematic trends at the rural sites and random variation at the urban site. The systematic trends qualitatively agree with a simple binary mixture model of secondary organic aerosol and background LV-WSOC that has delta13C of -17 permil or higher and f44 of approximately 0.06, respectively. Comparison with references suggests that the source of background LV-WSOC is likely biomass burning of C4 plants.